An electric power source arrangement as stated above, i.e. a fuel cell system capable of reducing an electric power loss, is known from U.S. Pat. No. 6,255,008 B1. In this system, a secondary battery is connected in parallel to the power source arrangement. Depending on the power demand of a load and the status of charge of the battery, the battery is connected or disconnected to the load, the fuel cell is connected to the load via the DC-DC voltage converter or via the by-pass branch or the fuel cell is connected to both the battery and load via the DC-DC voltage converter.
The characteristic of a fuel cell is that the ideal fuel cell has a comparably high output voltage which decreases when current is drawn. Then for a considerable interval of amperage the output voltage keeps almost constant, decreasing by some percent only, and then, after a certain value of current is passed, the voltage drops significantly. The DC-DC voltage converter serves to see to more or less constant voltage, at least in the steeper parts of the characteristics. It has an efficiency of e.g. 70% to 90%. The by-pass branch serves to avoid the loss caused by the converter as long as the voltage is sufficiently close to the nominal voltage.